Shape evolution in the neutron-rich osmium isotopes: Prompt gamma-ray spectroscopy of Os196.

The shape transition in the neutron-rich Os isotopes is studied by investigating the neutron-rich196Os nucleus through in-beam γ-ray spectroscopy using a two-proton transfer reaction from a 198Pt target to a 82Se beam. The beam-like recoils were detected and identified with the large-acceptance magnetic spectrometer PRISMA, and the coincident γ rays were measured with the advanced gamma tracking array (AGATA) demonstrator. The de-excitation of the low-lying levels of the yrast-band of 196Os were identified for the first time. The results are compared with state-of-the-art beyond-mean-field calculations, performed for the even-even 188−198Os isotopes. The new results suggest a smooth transition in the Os isotopes from a more axial rotational behavior towards predominately vibrational nuclei through triaxial configurations. An almost perfect γ -unstable/triaxial rotor yrast band is predicted for 196 Os which is in agreement with the experimentally measured excited states.